The inventive solution for a measuring head clamp-on device for ultrasonic flow-measuring heads lies within the technical field of ultrasonic flow measuring technology.
A pair of ultrasonic measuring heads is placed onto the outside of a tubing. By means of a measuring transducer, an ultrasonic pulse is generated alternately in both measuring heads and, after having passed through the tube wall and the medium in the tube, is received by the respectively other measuring head. Via the measured transit time difference (with or counter to the flow of the medium), the tube parameters and the flow profile, the flow quantity is calculated in the measuring transducer.
A precondition for these measurements is a secure transmission of the ultrasonic pulses from the measuring head into the tube wall. This is achieved by a suitable coupling means, the aligned orientation of the measuring heads, along the tube axis relative to one another, and by the contact pressing of the measuring heads onto the tube wall. The orientation of the measuring heads and the contact pressing are the functions of a measuring head clamp-on device.
The ultrasonic measuring heads are currently generally fitted by means of rail-like fastening systems, these being fastened to the tube with metallic band clamps or chains.
In addition to the rail-like systems for receiving one or more measuring head pairs, there are also fastening systems having receiving fixtures for receiving a single measuring head. The measuring heads are positioned by the fastening systems, and in most cases the measuring heads are pressed by means of spring elements elastically against the tube wall.
For instance, measuring head fastenings for ultrasonic flow meters, which fastenings guarantee a defined contact pressing force, are known. The measuring head fastenings are designed for fixed installation and offer a continuous, constant contact pressing of the measuring heads onto the tube.
In clamp-on flow measurement, the two sound transducers which are necessary to produce a sound path are arranged on the tube such that the sound path runs on the diameter of the tube. To this end, the sound transducers can be arranged on opposite sides of the tube. This is then the so-called direct arrangement (DE 102011005170 B4, DE 102005047790 A1). Alternatively, the reflex arrangement is possible, in which the sound transducers are arranged on the same side of the measuring tube and the sound signal is reflected once on the opposite inner side of the tube (DE 102008029772 A1).
In DE 102008034411 A1, a method and a measuring system for determining and/or monitoring the flow of a measuring medium through a measuring tube, comprising at least two ultrasonic measuring heads, is described. In addition, a method for fitting such a measuring system is disclosed. To this end, the measuring system has a measuring head holder, which can be detachably fastened to the measuring tube via a fastening mechanism. The measuring head housing with the ultrasonic measuring heads forms a unit fastenable to the sensor holder and detachable from the sensor holder. In the course of the assembly, the ultrasonic measuring heads are pressed with a predefinable force onto the measuring tube. A fastening plate, as part of the sensor holder, has on the side facing towards the measuring tube a prismatic bulge, which can be created as a bend in the fastening plate. The fastening plate is fastened at its front and rear end directly to the measuring tube, by means of U-bolts or band clamps.
In Patent Specification U.S. Pat. No. 5,131,278 A, the sensor holder is fastened by means of band clamps on the measuring tube by means of nuts and bolts.
In U.S. Pat. No. 6,397,683 B1, the measuring sensors are fastened on a rail and, with the aid of two tube clips, on the measuring tube. In order to adjust the distance between the two sensors, at least one sensor can be released from the tubing. Likewise, its fixing strap can be released. Both can be refixed after a reorientation.
EP 1840528 B1 discloses a rail for the displaceable fastening of ultrasonic measuring heads on the measuring tube.
In DE 102005052550 B3, a fastening device for fastening a holding device, for holding the measuring transducer, to the support is disclosed. The fastening device consists of a ratchet device and a main body, which latter is fastened with a knurled nut on the ratchet device. A centering apparatus consists of a groove into which a boss can be guided in order to be able to orient a guide frame securely in the longitudinal direction relative to the tube.
In the same way, U.S. Pat. No. 5,001,936 A shows that the mounting is provided with two legs, which do not readily allow adaptation to all possible tube diameters. In US 20120085164 A1 is represented a view from which it is evident that the smaller is the tube diameter, the more tightly will the measuring head rest on the tube. The guide element cannot be variably oriented.
In DE 102004052489 A1 is disclosed an apparatus which enables the sensor to be pressed directly against the tubing. For this, a coupling mat made of an elastomer or a metal foil is provided between the sensor and the tubing. The design serves to enable an increased contact pressure of the sensor.
WO 1998017980 A1 shows the possibility of adapting the spacing of the measuring device in accordance with the curvature of the tube by the use of spacers. The spacers are provided with pins and can be inserted at different positions prior to the clamping device being fitted onto the tube. A certain adaptation to different tube diameters is thereby possible, though a stepless adaptation is not provided.
In particular, the measurement of gases places an increased requirement on a sufficiently large measuring head pressing, since in this case damping elements are fitted to the tube walls in order to minimize background noise in the tube wall. In order to achieve a sufficiently good signal coupling despite these damping elements, a sufficiently high contact pressure of the measuring heads against the tube is in this case particularly important.
Most existing solutions fail to meet the requirement for a defined contact pressing force which is achieved irrespective of the know-how and skill of the user. If the user, for instance, fastens the measuring head clamp-on device with too little contact pressing force, a poor to inadequate coupling of the measuring heads to the tube is realized and no good signal coupling results therefrom. Or the measuring head clamp-on device is fastened with a too high contact pressing force, resulting in a too great load for the components used.